So, a “masque” is a form of entertainment for Renaissance era nobility. Kinda of like a costume party or an elaborate show of some kind.
Maybe the serial will actually feature one!
Of course it will. But first, the Doctor needs to escape the Brethren. They did do him a favor by vaporizing the Count, but they could do that to the Doctor, too.
Eh, he just dances in a circle around the altar with them and slipped out as soon as he could.
Back to see Sarah Jane and the Duke, and he has a plan. All he needs is a metal breastplate and a length of wire. Fortunately, both of these things exist. The Duke, meanwhile, is unsure if he should have his celebratory masque. It’s dangerous for his guests, but could be a show of weakness if he cancels it. The Doctor tells him to hold it. He asks Sarah Jane to stay with the Duke while he goes down to confront Hieronymous.
Or, more accurately, the Helix. Since, you know, that thing is in charge now.
Now, Sarah Jane, she’s worried. See, it’s scarier when the Doctor isn’t around than it is when he’s there. But at least it gets a little less scary when the Brethren goons show up and start vaporizing the Duke’s guests. I hope there weren’t any historically important people there. But then Hieronymous shows up, orders the Duke, Sarah Jane, and some others taken away, and off they go to chant around the altar some more.
But it turns out it was the Doctor disguised as Hieronymous. All the Brethren disintegrate because he’d wrapped a wire around the altar, sucking all the energy up as they used more and more. He’d already taunted Hieronymous into doing the same thing, and that breastplate kept the Doctor alive, plus…
…wait, did the Doctor just kill all those guys?…
Never mind. The Renaissance is saved! Sure, the Doctor never got to meet Leonardo Da Vinci, but we can’t have everything in life. While the Duke seems down to see the Doctor and Sarah Jane go, the Helix won’t be coming back. No, as the Doctor revs up the TARDIS, the Helix won’t be back for 500 years. That would be around the end of the 20th century. The Doctor says that was an interesting time.
Why? Well, he won’t say that much.
Next serial apparently is the last one to feature Sarah Jane as a regular companion, at least during the “classic” Doctor Who era.